The present invention relates to high efficiency air filters and, more specifically, to attachment of filter media to a support frame for use in high temperature application.
Air filters of the so-called "absolute" type are required to operate with a minimum 99.97% efficiency in filtering the air passing therethrough. In a popular construction of such filters, a continuous sheet of filter media is folded in zig-zag fashion to form a plurality of accordian pleats of uniform size and corrugated, thin corrugated sheets are placed between the opposing walls of each pleat on both the upstream and downstream sides to insure that the entire media surface is available for passage of air. The filter core, comprising the pleated media and spacers, is supported by a surrounding, box-like frame with which the media is sealingly engaged about its entire periphery, including both ends and the entire lengths of both pleat edges.
In some applications, filters of this type are installed in locations where they are subject to high temperatures, e.g., on the order of 1000.degree. F. One method of sealing the media to the frame involves the use of resin adhesives which are initially applied in liquid form and harden to form the necessary seal between the media and frame such as disclosed, for example, in U.S. Pat. No. 3,581,479 of Roger T. Goulet, issued June 1, 1971, and in application Ser. No. 856,674 of Stanley J. Wasielewski, filed of even date herewith, both assigned to applicant's assignee. However, such adhesives will not normally withstand temperatures as high as that mentioned above without destroying the effectiveness of the seal, thus requiring replacement of the filter unit. Therefore, other sealing means which remain effective during and after exposure to high temperatures, must be used in such applications but are difficult to provide.
One of the most successful structures providing the required sealing between the filter core and frame which is capable of withstanding high temperatures is that disclosed in U.S. Pat. No. 2,884,091. In such filter constructions a mat or blanket of fine glass fibers is compressed between the filter core and the surrounding metal frame. All components of the filter unit may be exposed to high temperature without danger of deterioration and the uniform compression of the glass fiber mat about the filter core provides a most effective seal, preventing bypass of unfiltered air around the filter media. However, at elevated temperatures the glass fibers begin to anneal causing the fibrous mat to loose resiliency resulting in leakage between core and frame. It is thus apparent that, although the sealing means itself may be efficient for the intended purpose, at extremely or even at moderately high temperatures approaching 1000.degree. F. filtering efficiency may be seriously reduced.
Accordingly, it is a principal object of the present invention to provide a method of fabricating a high-efficiency air filter unit wherein the sealing means which prevents bypass of the filter media is fully effective and retains its efficiency at high temperatures.
A further object is to provide an air filter fabrication method utilizing a ceramic adhesive and a ceramic fibrous mat to seal the core to the frame and, although the expansion coefficients vary between ceramic sealed core and frame, the fibrous mat maintains a seal between elements.
Other objects will in part be obvious and will in part appear hereinafter.